Better integration with Ctrl+Alt+Drag (column selection mode)

Hi Ala,

I have made a couple of changes to better integrate with the built-in column selection mode of Visual Studio.

Previously when using Ctrl+Alt+Drag to create a column selection and then typing text, the characters would (sometimes) be added multiple times on each line and each keystroke. This was because the built-in column selection mode and MultiEdit were both intercepting and repeating the keystrokes.

I have made a change so that a column selection with a null width is now converted into multiple edit points, replacing the default behavior. Since we do not (yet) support multiple selections, I have kept the default behavior for column selections with a width greater than 0.

Also the new behavior is better because it allows moving the caret around and not just typing text like the default does.

It would be great if you could test it and let me know your thoughts.

I still have the other task in mind (split selection) but unfortunately I haven't gotten around to completing it, been pretty busy lately.

Cheers
Xavier